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This CL
-Updates the multi-channel processing activation flags to match the
 the intended use in WebRTC.
-Makes sure that the APM in the audio processor complies to properly
run capture-side multi-channel processing.

Note: This CL can only land once the WebRTC CL
https://webrtc-review.googlesource.com/c/src/+/160707
has been landed and imported into Chromium.

Bug: webrtc:10859
Change-Id: Ied2b185d36e43187d90b5d608243702ec929ec1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1937030
Commit-Queue: Per Åhgren <peah@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719944}